PDA

View Full Version : Apo or not apo, that is the question...



Paul Schilliger
11-Jul-2004, 11:30
I have this lens, a Tessar 4,5 210 Docter Wetzlar 8092 mounted in current Copal 3. From the reflections of a light bulb on the cells which are all light pink, I would say it is single coated. Although this lens seems to have been produced in the last decade or two, it is named Tessar and not Apo-Tessar. Apo-Tessars seem to have been produced for a long time, and I am not sure that special glass was used to build the early ones. So would this sample not be Apo or is it simply that all modern Tessars were Apo anyway and the prefix was dropped by the terminologists of Docter Wetzlar?

Jim Rice
11-Jul-2004, 11:53
There was a time (say the 50's or 60's) when an apo designation mean't somthing. Only the pre-press folks cared. There was a Zeiss lens for the 'blad about that time that was called apo. With modern lenses, I doubt very seriously that chromatic aberation is much of an issue. OTOH, I love my apo-artar, but it was made when I was two.

Arne Croell
11-Jul-2004, 12:18
Paul, Apo-Tessars were process lenses with a maximum opening of f/9, the predecessors of the Apo-Germinars. After the introduction of the latter, Zeiss Jena phased out the Apo-Tessar in the mid-sixties. No Apo-Tessars were built after that. Apo-Tessars were highly corrected for process applications (1:1, but they work well at infinity), but only for a small angle of view, 43 was the official number. The regular Tessar that you have has a coverage around 55, plus you get the f/4.5 opening. If you go much over the stated 43 for the Apo-Tessar, astigmatism rears its ugly head - even according to Zeiss Jena itself (I read some test reports at their archives in Jena). Yes, the Tessars are single coated, even the Docter versions, but it doesn't matter that much with only 3 groups. You have to go for the M-Nikkors if you want a multicoated LF Tessar-type. The Apo-Germinar (single coated) or similar Apo-Ronar (single coated or multicoated) give you as good a performance (in terms of resolution with slightly reduced contrast) as any Apo-Tessar and over a slightly larger angle of coverage- 46-48.

Arne Croell
11-Jul-2004, 12:20
I forgot to answer the question directly. The Docter Tessars are not "Apo"! They are still very good lenses, regardless of that designation. The one drawback is that nearly all need a size 3 shutter...

Michael S. Briggs
11-Jul-2004, 12:21
Your Docter Wetzlar Tessar probably isn't apochromatic according to the traditional definition of bring three colors to the same focus. But the traditional definition was important for graphic arts / printing uses and isn't that important for pictorial use. What matters for photographic use is the overall correction over all colors, not just the three colors used in printing presses. In this era "Apo" has become a marketing term which the European manufacturers use not according to the traditional definition. The old LF Apo-Tessar was a slow lens so your current lens is probably more useful for general photography.

If you are happy with the images from your 210 mm plain Tessar, why switch? I wouldn't switch because the lens lacks "Apo" in the name. Reasons that you might consider a different lens: smaller/lighter, more coverage, ....

Paul Schilliger
11-Jul-2004, 12:56
Thanks for your enlightments! So you reminded me that the Apo design of the Apo-Tessars and other process lenses such as Apo-Ronars -Nikkors -Artars -Germinars was developped for the sake of graphic arts, which probably means these lenses are apo corrected at short distances but not at infinity anyway, although I know most will perform quite well there too. This Tessar is not part of my field equippment, for it is too heavy, but it is one of my favorites for table tops and other interior shots were the luminosity makes it easy to compose with and focus. I wouldn't qualify the lens of razor sharp, but it is quite good and the contrast is very natural. Should this lens cover 5x7 with some room for adjustments? I have numbers of 210 or 250mm for the image circle.

Arne Croell
11-Jul-2004, 13:06
Paul, your supposition that the Apo correction has been done strictly only around 1:1 is correct. That doesn't mean that all of these lenses can't perform beautifully at infinity, as you noted. Wide open the Tessar is not razor sharp as you said (but good for portraits, without being a soft-focus lens), but it improves a lot at f/11 or lower. The angle of coverage given by Docter is 55, which translates into 219mm as image circle. At the same time, the Docter literature lists the 250mm you quoted. Take your pick... Both are sufficient for 5x7, although not with much movements. I have Docters MTF curves, if you're interested I can check there.

Paul Schilliger
11-Jul-2004, 13:28
Arne, thanks for the latest! BTW I just checked a scan from a Tessar shot and if it is not razor sharp, it resembles it! And of course there is not the slightest color fringing on the edges of the silver cutlery as you would expect from any modern lens.

Arne Croell
11-Jul-2004, 13:46
When checking, note that a characteristic feature of Tessars is that their MTF curves start out high, then go down to a first minimum in performance about halfway to 2/3rd of the image circle and then go back up to high values before finally falling down. An example can be seen <a href="http://www.schneideroptics.com/photography/large_format_lenses/xenar/pdf/xenar_61_210.pdf"> here<a> for a Xenar. The Docter Tessar curves have the same characteristic. This is more pronounced at larger apertures, and stopping down essentially pulls up that intermediate part with less contrast. So when evaluating pictures taken at larger apertures it is worthwhile to note where that initial part with a low MTF would be. Without movements, my guess would be that for your 210mm that part is at or a little beyond the corners of 4x5.

Paul Schilliger
11-Jul-2004, 14:44
Although I am not very good at reading MTF charts, this characteristic is pretty obvious on the drawing. It's good to know that the center of the image is crisp at wide aperture, it makes it a good lens for a sharp centred subject on a blurred background. Thanks for the link, Arne. BTW, I have this Xenar 6,1 210 and it is a good small lens for landscape.

Dan Fromm
11-Jul-2004, 16:19
Arne, could you be reading too much into Schneider's MTF charts for the 210/6.1 Xenar?

It seems to me that the big news from them isn't the minor dip in contrast part way out from the center of the image, but instead that stopped down to f/22 there's very low contrast near the edges even at low frequency. In other words, the lens doesn't really cover the format, even at f/22.

Cheers,

Dan

Arne Croell
12-Jul-2004, 01:25
Hi Dan, the falloff at the end is certainly there, but I think it is not a characteristic of the Xenar/Tesssar. Most normal (excluding process) lenses MTF curves look that way. Compare the MTF of the recent Apo-Symmar L 210mm <a href="http://www.schneideroptics.com/photography/large_format_lenses/apo-symmar-L/pdf/ApoSymmarL_56_210.pdf"> here<a> and at 100% and f22 there is not much there either - only the tangential curves are higher than the sagittal ones in this case which is reverse to the Xenar but doesn't change the overall quality. One should use the full coverage only in an emergency anyway. At f/22 the dip is minor and similar curves can be seen for Plasmat types. My point was that when used at _wider_ apertures it is quite pronounced and should be taken into account. For the 210mm f/4.5 Tessar Paul is asking about I have the curves for f/8 and 2.5 and 5 lp/mm. The 5 lp/mm sagittal curve starts at about 94% MTF, the dip goes down to 65% (at 55% image height), and the second maximum goes back up to 92%MTF (at 96% imgage height!). A difference of nearly 30% in MTF is noticeable.
Paul, Docter recommended f/22 fo best results, btw.